| River is an important link of the material flux between continent and ocean.It is closely related to human civilization,culture and history.With economic globalization,the trend of heavy metal pollution in rivers is spreading worldwide.Since heavy metals discharged into rivers tend to enrich in sediment and belong to cumulative toxicant,they can reflect the difference of interference degree of human activities in rivers in different regions by observing and analyzing their accumulation degrees in riverbed sediments from different parts of the world.In addition,the contents of heavy metals in sediments can also be used as indicators to evaluate the environmental quality of rivers and to trace the sources of pollution.In assessing the risk of riverbed sediments,researchers usually compare the measured concentrations of heavy metals in sediments from the study area with geochemical background values or background values from non-polluted samples.However,due to the intervention of human activities,it is difficult to find natural pollution-free samples that reflect the natural state of background.Meanwhile,many research areas do not have regional geochemical background values or ambient background values.To fill this gap,this study summarized 304 available literatures involving a total of 7583 sampling sites in 54 countries in all continents of the world,built a basic database containing the distribution of average,maximum and minimum contents of nine target heavy metals(Cr,Ni,Cu,Zn,As,Cd,Pb,Sb and Tl)in riverbed sediments from five regions of the world,and then,according to the calculation method of background value,proposed a set of data processing procedures,including data structure observation,concentration threshold calculation and distribution level revision,to obtain the world/national wide ambient background and threshold values of target metals.On this basis,the regional economy,industrialization level,mineral resources endowment,mining intensity and other factors were combined with above statistical results to explore the possible causes of the regional distribution differences of heavy metals.On the other hand,the distribution of heavy metals in riverbed sediments is the result of the outcome of internal factors(e.g.,environment factors),which determine the existing forms and migration abilities of heavy metals,and external factors(e.g.,the locations and intensities of emission sources),which determine the accumulation degrees of metals.However,although many researches have successfully established the connections between metal distribution and internal-external factors by using statistical methods,those methods focus on studying the connections between two variables(e.g.,correlation analysis)or the possible connections among variables in a dataset(e.g.,canonical correspondence analysis).No reports were found about the use of statistical method which quantify the strength of relationship between a set of internal-external factors and a set of metal contents.To fulfill above objectives,this study proposed a series of statistical operations,collectively called optimized CCA analysis,which is on the basis of canonical correlation analysis(CCA analysis)and combined with many other multivariate statistical analysis methods for index optimization,to quantify the strength of relationship between integrated effect of internal-external factors and multiple metal distribution in riverbed sediments.To demonstrate this procedure,the middle and lower reach of Xijiang River basin,which flows through Hechi,Guangxi to Zhaoqing,Guangdong,were chosen as the area for study.Riverbed sediment samples were collected in April and October 2015,respectively.And then,the total contents,geochemical phases and acid-leachable fraction of heavy metals and the physical-chemical characteristics of sediments(defined as internal factors)were determined.The distribution and risk of heavy metals in surface sediments of study area were evaluated by using a variety of pollution assessment methods based on world/national wide threshold values,regional background values and bioavailability of heavy metals in sediments.In addition,statistical indicators reflecting human activities(defined as external factors)were collected.Finally,optimized CCA analysis was applied to quantify the canonical correlation between integrated effect of internal-external factors and multiple metal distribution,and to identify the characteristic parameters in external/internal factors which significantly related with metal distributions.Combined with the characteristic parameters and the distribution of multiple metals and their risks,some river basin management recommendations for sub target objects and sub regions were proposed.The main results are listed as follows:? World/national wide threshold values(including concentration distribution grade values,threshold values and ambient background values)for 9 target metals have been successfully obtained.They can be used as a basis for evaluating the relative pollution level of heavy metals in riverbed sediments and as reference values for evaluating the concentration of heavy metals in sediments which were generally polluted.? Owing to their early development in industrial,the extents of river pollution in developed countries is generally higher than that in developing countries.In addition,because of the migration of pollution-intensive enterprises to developing countries and the distribution and exploitation of mineral resources in the world are mainly in developing countries,developing countries have greater potential for heavy metal pollution of rivers to deteriorate in future.On the other hand,the distribution characteristics of China's industrial and mineral resources determine that the types and extents of heavy metal pollutions in sediments of the Yangtze River,Pearl River and eastern coastal rivers are higher than those rivers in other regions.Compared with the pollution level of the world's general rivers,Cr,Ni,Zn and As in riverbed sediments of China are enriched,meanwhile,Cd is the most enrichment and widely distributed polluted element.? Compared with the world/national wide threshold values,Zn,As,Cd,Pb and Sb in the sediments of the middle and lower reach of Xijiang River basin were relatively polluted.And the extremely serious pollution of Cd and Sb must arouse great attention.In addition,Cd,Sb and Tl have the highest risk than other metals,and the bioavailability of heavy metals decreased after the rainy season.Heavy metal pollution in Longjiang River basin was significantly higher than that in other river sections.? The results of optimized CCA analysis show that,for those metals in natural states,the affinity of sediment(e.g.,Fe and Mn oxides)is responsible for their distributions.By contrast,when metals had obvious anthropogenic sources,the interferences of anthropogenic inputs(e.g.,non-ferrous metal enterprises' waste-discharging activities)and the specific sedimentary characteristics(e.g.karst topography and low-energy hydrodynamic depositional conditions)in study area can weaken the correlation between the binding affinities of sediment and the contents of heavy metals.By successfully identifying the major internal/external factors which significantly related with multiple metal distributions in sediments of study area,it is proved that the optimized CCA analysis can be an alternative and advantageous statistical operation of correlation analysis.In summary,this study obtained the world/national wide threshold values for 9 target metals which can provide reference values for river health assessment.Meanwhile,this study proposed a series of statistical operations,collectively called optimized CCA analysis,to quantify the strength of relationship between integrated effect of internal-external factors and multiple metal distribution in riverbed sediments.This statistical analysis method can be extended to other research areas or pollutants,and is a new tool for the diagnosis of compound pollution of rivers. |
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